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PERLXS(1)			 Perl Programmers Reference Guide			PERLXS(1)

NAME
       perlxs - XS language reference manual

DESCRIPTION
       Introduction

       XS is an interface description file format used to create an extension interface between
       Perl and C code (or a C library) which one wishes to use with Perl.  The XS interface is
       combined with the library to create a new library which can then be either dynamically
       loaded or statically linked into perl.  The XS interface description is written in the XS
       language and is the core component of the Perl extension interface.

       An XSUB forms the basic unit of the XS interface.  After compilation by the xsubpp com-
       piler, each XSUB amounts to a C function definition which will provide the glue between
       Perl calling conventions and C calling conventions.

       The glue code pulls the arguments from the Perl stack, converts these Perl values to the
       formats expected by a C function, call this C function, transfers the return values of the
       C function back to Perl.  Return values here may be a conventional C return value or any C
       function arguments that may serve as output parameters.	These return values may be passed
       back to Perl either by putting them on the Perl stack, or by modifying the arguments sup-
       plied from the Perl side.

       The above is a somewhat simplified view of what really happens.	Since Perl allows more
       flexible calling conventions than C, XSUBs may do much more in practice, such as checking
       input parameters for validity, throwing exceptions (or returning undef/empty list) if the
       return value from the C function indicates failure, calling different C functions based on
       numbers and types of the arguments, providing an object-oriented interface, etc.

       Of course, one could write such glue code directly in C.  However, this would be a tedious
       task, especially if one needs to write glue for multiple C functions, and/or one is not
       familiar enough with the Perl stack discipline and other such arcana.  XS comes to the
       rescue here: instead of writing this glue C code in long-hand, one can write a more con-
       cise short-hand description of what should be done by the glue, and let the XS compiler
       xsubpp handle the rest.

       The XS language allows one to describe the mapping between how the C routine is used, and
       how the corresponding Perl routine is used.  It also allows creation of Perl routines
       which are directly translated to C code and which are not related to a pre-existing C
       function.  In cases when the C interface coincides with the Perl interface, the XSUB dec-
       laration is almost identical to a declaration of a C function (in K&R style).  In such
       circumstances, there is another tool called "h2xs" that is able to translate an entire C
       header file into a corresponding XS file that will provide glue to the functions/macros
       described in the header file.

       The XS compiler is called xsubpp.  This compiler creates the constructs necessary to let
       an XSUB manipulate Perl values, and creates the glue necessary to let Perl call the XSUB.
       The compiler uses typemaps to determine how to map C function parameters and output values
       to Perl values and back.  The default typemap (which comes with Perl) handles many common
       C types.  A supplementary typemap may also be needed to handle any special structures and
       types for the library being linked.

       A file in XS format starts with a C language section which goes until the first "MODULE ="
       directive.  Other XS directives and XSUB definitions may follow this line.  The "language"
       used in this part of the file is usually referred to as the XS language.  xsubpp recog-
       nizes and skips POD (see perlpod) in both the C and XS language sections, which allows the
       XS file to contain embedded documentation.

       See perlxstut for a tutorial on the whole extension creation process.

       Note: For some extensions, Dave Beazley's SWIG system may provide a significantly more
       convenient mechanism for creating the extension glue code.  See http://www.swig.org/ for
       more information.

       On The Road

       Many of the examples which follow will concentrate on creating an interface between Perl
       and the ONC+ RPC bind library functions.  The rpcb_gettime() function is used to demon-
       strate many features of the XS language.  This function has two parameters; the first is
       an input parameter and the second is an output parameter.  The function also returns a
       status value.

	       bool_t rpcb_gettime(const char *host, time_t *timep);

       From C this function will be called with the following statements.

	    #include <rpc/rpc.h>
	    bool_t status;
	    time_t timep;
	    status = rpcb_gettime( "localhost", &timep );

       If an XSUB is created to offer a direct translation between this function and Perl, then
       this XSUB will be used from Perl with the following code.  The $status and $timep vari-
       ables will contain the output of the function.

	    use RPC;
	    $status = rpcb_gettime( "localhost", $timep );

       The following XS file shows an XS subroutine, or XSUB, which demonstrates one possible
       interface to the rpcb_gettime() function.  This XSUB represents a direct translation
       between C and Perl and so preserves the interface even from Perl.  This XSUB will be
       invoked from Perl with the usage shown above.  Note that the first three #include state-
       ments, for "EXTERN.h", "perl.h", and "XSUB.h", will always be present at the beginning of
       an XS file.  This approach and others will be expanded later in this document.

	    #include "EXTERN.h"
	    #include "perl.h"
	    #include "XSUB.h"
	    #include <rpc/rpc.h>

	    MODULE = RPC  PACKAGE = RPC

	    bool_t
	    rpcb_gettime(host,timep)
		 char *host
		 time_t &timep
	       OUTPUT:
		 timep

       Any extension to Perl, including those containing XSUBs, should have a Perl module to
       serve as the bootstrap which pulls the extension into Perl.  This module will export the
       extension's functions and variables to the Perl program and will cause the extension's
       XSUBs to be linked into Perl.  The following module will be used for most of the examples
       in this document and should be used from Perl with the "use" command as shown earlier.
       Perl modules are explained in more detail later in this document.

	    package RPC;

	    require Exporter;
	    require DynaLoader;
	    @ISA = qw(Exporter DynaLoader);
	    @EXPORT = qw( rpcb_gettime );

	    bootstrap RPC;
	    1;

       Throughout this document a variety of interfaces to the rpcb_gettime() XSUB will be
       explored.  The XSUBs will take their parameters in different orders or will take different
       numbers of parameters.  In each case the XSUB is an abstraction between Perl and the real
       C rpcb_gettime() function, and the XSUB must always ensure that the real rpcb_gettime()
       function is called with the correct parameters.	This abstraction will allow the program-
       mer to create a more Perl-like interface to the C function.

       The Anatomy of an XSUB

       The simplest XSUBs consist of 3 parts: a description of the return value, the name of the
       XSUB routine and the names of its arguments, and a description of types or formats of the
       arguments.

       The following XSUB allows a Perl program to access a C library function called sin().  The
       XSUB will imitate the C function which takes a single argument and returns a single value.

	    double
	    sin(x)
	      double x

       Optionally, one can merge the description of types and the list of argument names, rewrit-
       ing this as

	    double
	    sin(double x)

       This makes this XSUB look similar to an ANSI C declaration.  An optional semicolon is
       allowed after the argument list, as in

	    double
	    sin(double x);

       Parameters with C pointer types can have different semantic: C functions with similar dec-
       larations

	    bool string_looks_as_a_number(char *s);
	    bool make_char_uppercase(char *c);

       are used in absolutely incompatible manner.  Parameters to these functions could be
       described xsubpp like this:

	    char *  s
	    char    &c

       Both these XS declarations correspond to the "char*" C type, but they have different
       semantics, see "The & Unary Operator".

       It is convenient to think that the indirection operator "*" should be considered as a part
       of the type and the address operator "&" should be considered part of the variable.  See
       "The Typemap" for more info about handling qualifiers and unary operators in C types.

       The function name and the return type must be placed on separate lines and should be flush
       left-adjusted.

	 INCORRECT			  CORRECT

	 double sin(x)			  double
	   double x			  sin(x)
					    double x

       The rest of the function description may be indented or left-adjusted. The following exam-
       ple shows a function with its body left-adjusted.  Most examples in this document will
       indent the body for better readability.

	 CORRECT

	 double
	 sin(x)
	 double x

       More complicated XSUBs may contain many other sections.	Each section of an XSUB starts
       with the corresponding keyword, such as INIT: or CLEANUP:.  However, the first two lines
       of an XSUB always contain the same data: descriptions of the return type and the names of
       the function and its parameters.  Whatever immediately follows these is considered to be
       an INPUT: section unless explicitly marked with another keyword.  (See "The INPUT: Key-
       word".)

       An XSUB section continues until another section-start keyword is found.

       The Argument Stack

       The Perl argument stack is used to store the values which are sent as parameters to the
       XSUB and to store the XSUB's return value(s).  In reality all Perl functions (including
       non-XSUB ones) keep their values on this stack all the same time, each limited to its own
       range of positions on the stack.  In this document the first position on that stack which
       belongs to the active function will be referred to as position 0 for that function.

       XSUBs refer to their stack arguments with the macro ST(x), where x refers to a position in
       this XSUB's part of the stack.  Position 0 for that function would be known to the XSUB as
       ST(0).  The XSUB's incoming parameters and outgoing return values always begin at ST(0).
       For many simple cases the xsubpp compiler will generate the code necessary to handle the
       argument stack by embedding code fragments found in the typemaps.  In more complex cases
       the programmer must supply the code.

       The RETVAL Variable

       The RETVAL variable is a special C variable that is declared automatically for you.  The C
       type of RETVAL matches the return type of the C library function.  The xsubpp compiler
       will declare this variable in each XSUB with non-"void" return type.  By default the gen-
       erated C function will use RETVAL to hold the return value of the C library function being
       called.	In simple cases the value of RETVAL will be placed in ST(0) of the argument stack
       where it can be received by Perl as the return value of the XSUB.

       If the XSUB has a return type of "void" then the compiler will not declare a RETVAL vari-
       able for that function.	When using a PPCODE: section no manipulation of the RETVAL vari-
       able is required, the section may use direct stack manipulation to place output values on
       the stack.

       If PPCODE: directive is not used, "void" return value should be used only for subroutines
       which do not return a value, even if CODE: directive is used which sets ST(0) explicitly.

       Older versions of this document recommended to use "void" return value in such cases. It
       was discovered that this could lead to segfaults in cases when XSUB was truly "void". This
       practice is now deprecated, and may be not supported at some future version. Use the
       return value "SV *" in such cases. (Currently "xsubpp" contains some heuristic code which
       tries to disambiguate between "truly-void" and "old-practice-declared-as-void" functions.
       Hence your code is at mercy of this heuristics unless you use "SV *" as return value.)

       Returning SVs, AVs and HVs through RETVAL

       When you're using RETVAL to return an "SV *", there's some magic going on behind the
       scenes that should be mentioned. When you're manipulating the argument stack using the
       ST(x) macro, for example, you usually have to pay special attention to reference counts.
       (For more about reference counts, see perlguts.) To make your life easier, the typemap
       file automatically makes "RETVAL" mortal when you're returning an "SV *". Thus, the fol-
       lowing two XSUBs are more or less equivalent:

	 void
	 alpha()
	     PPCODE:
		 ST(0) = newSVpv("Hello World",0);
		 sv_2mortal(ST(0));
		 XSRETURN(1);

	 SV *
	 beta()
	     CODE:
		 RETVAL = newSVpv("Hello World",0);
	     OUTPUT:
		 RETVAL

       This is quite useful as it usually improves readability. While this works fine for an "SV
       *", it's unfortunately not as easy to have "AV *" or "HV *" as a return value. You should
       be able to write:

	 AV *
	 array()
	     CODE:
		 RETVAL = newAV();
		 /* do something with RETVAL */
	     OUTPUT:
		 RETVAL

       But due to an unfixable bug (fixing it would break lots of existing CPAN modules) in the
       typemap file, the reference count of the "AV *" is not properly decremented. Thus, the
       above XSUB would leak memory whenever it is being called. The same problem exists for "HV
       *".

       When you're returning an "AV *" or a "HV *", you have make sure their reference count is
       decremented by making the AV or HV mortal:

	 AV *
	 array()
	     CODE:
		 RETVAL = newAV();
		 sv_2mortal((SV*)RETVAL);
		 /* do something with RETVAL */
	     OUTPUT:
		 RETVAL

       And also remember that you don't have to do this for an "SV *".

       The MODULE Keyword

       The MODULE keyword is used to start the XS code and to specify the package of the func-
       tions which are being defined.  All text preceding the first MODULE keyword is considered
       C code and is passed through to the output with POD stripped, but otherwise untouched.
       Every XS module will have a bootstrap function which is used to hook the XSUBs into Perl.
       The package name of this bootstrap function will match the value of the last MODULE state-
       ment in the XS source files.  The value of MODULE should always remain constant within the
       same XS file, though this is not required.

       The following example will start the XS code and will place all functions in a package
       named RPC.

	    MODULE = RPC

       The PACKAGE Keyword

       When functions within an XS source file must be separated into packages the PACKAGE key-
       word should be used.  This keyword is used with the MODULE keyword and must follow immedi-
       ately after it when used.

	    MODULE = RPC  PACKAGE = RPC

	    [ XS code in package RPC ]

	    MODULE = RPC  PACKAGE = RPCB

	    [ XS code in package RPCB ]

	    MODULE = RPC  PACKAGE = RPC

	    [ XS code in package RPC ]

       The same package name can be used more than once, allowing for non-contiguous code. This
       is useful if you have a stronger ordering principle than package names.

       Although this keyword is optional and in some cases provides redundant information it
       should always be used.  This keyword will ensure that the XSUBs appear in the desired
       package.

       The PREFIX Keyword

       The PREFIX keyword designates prefixes which should be removed from the Perl function
       names.  If the C function is "rpcb_gettime()" and the PREFIX value is "rpcb_" then Perl
       will see this function as "gettime()".

       This keyword should follow the PACKAGE keyword when used.  If PACKAGE is not used then
       PREFIX should follow the MODULE keyword.

	    MODULE = RPC  PREFIX = rpc_

	    MODULE = RPC  PACKAGE = RPCB  PREFIX = rpcb_

       The OUTPUT: Keyword

       The OUTPUT: keyword indicates that certain function parameters should be updated (new val-
       ues made visible to Perl) when the XSUB terminates or that certain values should be
       returned to the calling Perl function.  For simple functions which have no CODE: or
       PPCODE: section, such as the sin() function above, the RETVAL variable is automatically
       designated as an output value.  For more complex functions the xsubpp compiler will need
       help to determine which variables are output variables.

       This keyword will normally be used to complement the CODE:  keyword.  The RETVAL variable
       is not recognized as an output variable when the CODE: keyword is present.  The OUTPUT:
       keyword is used in this situation to tell the compiler that RETVAL really is an output
       variable.

       The OUTPUT: keyword can also be used to indicate that function parameters are output vari-
       ables.  This may be necessary when a parameter has been modified within the function and
       the programmer would like the update to be seen by Perl.

	    bool_t
	    rpcb_gettime(host,timep)
		 char *host
		 time_t &timep
	       OUTPUT:
		 timep

       The OUTPUT: keyword will also allow an output parameter to be mapped to a matching piece
       of code rather than to a typemap.

	    bool_t
	    rpcb_gettime(host,timep)
		 char *host
		 time_t &timep
	       OUTPUT:
		 timep sv_setnv(ST(1), (double)timep);

       xsubpp emits an automatic "SvSETMAGIC()" for all parameters in the OUTPUT section of the
       XSUB, except RETVAL.  This is the usually desired behavior, as it takes care of properly
       invoking 'set' magic on output parameters (needed for hash or array element parameters
       that must be created if they didn't exist).  If for some reason, this behavior is not
       desired, the OUTPUT section may contain a "SETMAGIC: DISABLE" line to disable it for the
       remainder of the parameters in the OUTPUT section.  Likewise,  "SETMAGIC: ENABLE" can be
       used to reenable it for the remainder of the OUTPUT section.  See perlguts for more
       details about 'set' magic.

       The NO_OUTPUT Keyword

       The NO_OUTPUT can be placed as the first token of the XSUB.  This keyword indicates that
       while the C subroutine we provide an interface to has a non-"void" return type, the return
       value of this C subroutine should not be returned from the generated Perl subroutine.

       With this keyword present "The RETVAL Variable" is created, and in the generated call to
       the subroutine this variable is assigned to, but the value of this variable is not going
       to be used in the auto-generated code.

       This keyword makes sense only if "RETVAL" is going to be accessed by the user-supplied
       code.  It is especially useful to make a function interface more Perl-like, especially
       when the C return value is just an error condition indicator.  For example,

	 NO_OUTPUT int
	 delete_file(char *name)
	   POSTCALL:
	     if (RETVAL != 0)
		 croak("Error %d while deleting file '%s'", RETVAL, name);

       Here the generated XS function returns nothing on success, and will die() with a meaning-
       ful error message on error.

       The CODE: Keyword

       This keyword is used in more complicated XSUBs which require special handling for the C
       function.  The RETVAL variable is still declared, but it will not be returned unless it is
       specified in the OUTPUT: section.

       The following XSUB is for a C function which requires special handling of its parameters.
       The Perl usage is given first.

	    $status = rpcb_gettime( "localhost", $timep );

       The XSUB follows.

	    bool_t
	    rpcb_gettime(host,timep)
		 char *host
		 time_t timep
	       CODE:
		      RETVAL = rpcb_gettime( host, &timep );
	       OUTPUT:
		 timep
		 RETVAL

       The INIT: Keyword

       The INIT: keyword allows initialization to be inserted into the XSUB before the compiler
       generates the call to the C function.  Unlike the CODE: keyword above, this keyword does
       not affect the way the compiler handles RETVAL.

	   bool_t
	   rpcb_gettime(host,timep)
		 char *host
		 time_t &timep
	       INIT:
		 printf("# Host is %s\n", host );
	       OUTPUT:
		 timep

       Another use for the INIT: section is to check for preconditions before making a call to
       the C function:

	   long long
	   lldiv(a,b)
	       long long a
	       long long b
	     INIT:
	       if (a == 0 && b == 0)
		   XSRETURN_UNDEF;
	       if (b == 0)
		   croak("lldiv: cannot divide by 0");

       The NO_INIT Keyword

       The NO_INIT keyword is used to indicate that a function parameter is being used only as an
       output value.  The xsubpp compiler will normally generate code to read the values of all
       function parameters from the argument stack and assign them to C variables upon entry to
       the function.  NO_INIT will tell the compiler that some parameters will be used for output
       rather than for input and that they will be handled before the function terminates.

       The following example shows a variation of the rpcb_gettime() function.	This function
       uses the timep variable only as an output variable and does not care about its initial
       contents.

	    bool_t
	    rpcb_gettime(host,timep)
		 char *host
		 time_t &timep = NO_INIT
	       OUTPUT:
		 timep

       Initializing Function Parameters

       C function parameters are normally initialized with their values from the argument stack
       (which in turn contains the parameters that were passed to the XSUB from Perl).	The
       typemaps contain the code segments which are used to translate the Perl values to the C
       parameters.  The programmer, however, is allowed to override the typemaps and supply
       alternate (or additional) initialization code.  Initialization code starts with the first
       "=", ";" or "+" on a line in the INPUT: section.  The only exception happens if this ";"
       terminates the line, then this ";" is quietly ignored.

       The following code demonstrates how to supply initialization code for function parameters.
       The initialization code is eval'ed within double quotes by the compiler before it is added
       to the output so anything which should be interpreted literally [mainly "$", "@", or "\\"]
       must be protected with backslashes.  The variables $var, $arg, and $type can be used as in
       typemaps.

	    bool_t
	    rpcb_gettime(host,timep)
		 char *host = (char *)SvPV_nolen($arg);
		 time_t &timep = 0;
	       OUTPUT:
		 timep

       This should not be used to supply default values for parameters.  One would normally use
       this when a function parameter must be processed by another library function before it can
       be used.  Default parameters are covered in the next section.

       If the initialization begins with "=", then it is output in the declaration for the input
       variable, replacing the initialization supplied by the typemap.	If the initialization
       begins with ";" or "+", then it is performed after all of the input variables have been
       declared.  In the ";" case the initialization normally supplied by the typemap is not per-
       formed.	For the "+" case, the declaration for the variable will include the initializa-
       tion from the typemap.  A global variable, %v, is available for the truly rare case where
       information from one initialization is needed in another initialization.

       Here's a truly obscure example:

	    bool_t
	    rpcb_gettime(host,timep)
		 time_t &timep; /* \$v{timep}=@{[$v{timep}=$arg]} */
		 char *host + SvOK($v{timep}) ? SvPV_nolen($arg) : NULL;
	       OUTPUT:
		 timep

       The construct "\$v{timep}=@{[$v{timep}=$arg]}" used in the above example has a two-fold
       purpose: first, when this line is processed by xsubpp, the Perl snippet "$v{timep}=$arg"
       is evaluated.  Second, the text of the evaluated snippet is output into the generated C
       file (inside a C comment)!  During the processing of "char *host" line, $arg will evaluate
       to ST(0), and $v{timep} will evaluate to ST(1).

       Default Parameter Values

       Default values for XSUB arguments can be specified by placing an assignment statement in
       the parameter list.  The default value may be a number, a string or the special string
       "NO_INIT".  Defaults should always be used on the right-most parameters only.

       To allow the XSUB for rpcb_gettime() to have a default host value the parameters to the
       XSUB could be rearranged.  The XSUB will then call the real rpcb_gettime() function with
       the parameters in the correct order.  This XSUB can be called from Perl with either of the
       following statements:

	    $status = rpcb_gettime( $timep, $host );

	    $status = rpcb_gettime( $timep );

       The XSUB will look like the code  which	follows.   A  CODE: block  is used to call the
       real rpcb_gettime() function with the parameters in the correct order for that function.

	    bool_t
	    rpcb_gettime(timep,host="localhost")
		 char *host
		 time_t timep = NO_INIT
	       CODE:
		      RETVAL = rpcb_gettime( host, &timep );
	       OUTPUT:
		 timep
		 RETVAL

       The PREINIT: Keyword

       The PREINIT: keyword allows extra variables to be declared immediately before or after the
       declarations of the parameters from the INPUT: section are emitted.

       If a variable is declared inside a CODE: section it will follow any typemap code that is
       emitted for the input parameters.  This may result in the declaration ending up after C
       code, which is C syntax error.  Similar errors may happen with an explicit ";"-type or
       "+"-type initialization of parameters is used (see "Initializing Function Parameters").
       Declaring these variables in an INIT: section will not help.

       In such cases, to force an additional variable to be declared together with declarations
       of other variables, place the declaration into a PREINIT: section.  The PREINIT: keyword
       may be used one or more times within an XSUB.

       The following examples are equivalent, but if the code is using complex typemaps then the
       first example is safer.

	    bool_t
	    rpcb_gettime(timep)
		 time_t timep = NO_INIT
	       PREINIT:
		 char *host = "localhost";
	       CODE:
		 RETVAL = rpcb_gettime( host, &timep );
	       OUTPUT:
		 timep
		 RETVAL

       For this particular case an INIT: keyword would generate the same C code as the PREINIT:
       keyword.  Another correct, but error-prone example:

	    bool_t
	    rpcb_gettime(timep)
		 time_t timep = NO_INIT
	       CODE:
		 char *host = "localhost";
		 RETVAL = rpcb_gettime( host, &timep );
	       OUTPUT:
		 timep
		 RETVAL

       Another way to declare "host" is to use a C block in the CODE: section:

	    bool_t
	    rpcb_gettime(timep)
		 time_t timep = NO_INIT
	       CODE:
		 {
		   char *host = "localhost";
		   RETVAL = rpcb_gettime( host, &timep );
		 }
	       OUTPUT:
		 timep
		 RETVAL

       The ability to put additional declarations before the typemap entries are processed is
       very handy in the cases when typemap conversions manipulate some global state:

	   MyObject
	   mutate(o)
	       PREINIT:
		   MyState st = global_state;
	       INPUT:
		   MyObject o;
	       CLEANUP:
		   reset_to(global_state, st);

       Here we suppose that conversion to "MyObject" in the INPUT: section and from MyObject when
       processing RETVAL will modify a global variable "global_state".	After these conversions
       are performed, we restore the old value of "global_state" (to avoid memory leaks, for
       example).

       There is another way to trade clarity for compactness: INPUT sections allow declaration of
       C variables which do not appear in the parameter list of a subroutine.  Thus the above
       code for mutate() can be rewritten as

	   MyObject
	   mutate(o)
		 MyState st = global_state;
		 MyObject o;
	       CLEANUP:
		 reset_to(global_state, st);

       and the code for rpcb_gettime() can be rewritten as

	    bool_t
	    rpcb_gettime(timep)
		 time_t timep = NO_INIT
		 char *host = "localhost";
	       C_ARGS:
		 host, &timep
	       OUTPUT:
		 timep
		 RETVAL

       The SCOPE: Keyword

       The SCOPE: keyword allows scoping to be enabled for a particular XSUB. If enabled, the
       XSUB will invoke ENTER and LEAVE automatically.

       To support potentially complex type mappings, if a typemap entry used by an XSUB contains
       a comment like "/*scope*/" then scoping will be automatically enabled for that XSUB.

       To enable scoping:

	   SCOPE: ENABLE

       To disable scoping:

	   SCOPE: DISABLE

       The INPUT: Keyword

       The XSUB's parameters are usually evaluated immediately after entering the XSUB.  The
       INPUT: keyword can be used to force those parameters to be evaluated a little later.  The
       INPUT: keyword can be used multiple times within an XSUB and can be used to list one or
       more input variables.  This keyword is used with the PREINIT: keyword.

       The following example shows how the input parameter "timep" can be evaluated late, after a
       PREINIT.

	   bool_t
	   rpcb_gettime(host,timep)
		 char *host
	       PREINIT:
		 time_t tt;
	       INPUT:
		 time_t timep
	       CODE:
		      RETVAL = rpcb_gettime( host, &tt );
		      timep = tt;
	       OUTPUT:
		 timep
		 RETVAL

       The next example shows each input parameter evaluated late.

	   bool_t
	   rpcb_gettime(host,timep)
	       PREINIT:
		 time_t tt;
	       INPUT:
		 char *host
	       PREINIT:
		 char *h;
	       INPUT:
		 time_t timep
	       CODE:
		      h = host;
		      RETVAL = rpcb_gettime( h, &tt );
		      timep = tt;
	       OUTPUT:
		 timep
		 RETVAL

       Since INPUT sections allow declaration of C variables which do not appear in the parameter
       list of a subroutine, this may be shortened to:

	   bool_t
	   rpcb_gettime(host,timep)
		 time_t tt;
		 char *host;
		 char *h = host;
		 time_t timep;
	       CODE:
		 RETVAL = rpcb_gettime( h, &tt );
		 timep = tt;
	       OUTPUT:
		 timep
		 RETVAL

       (We used our knowledge that input conversion for "char *" is a "simple" one, thus "host"
       is initialized on the declaration line, and our assignment "h = host" is not performed too
       early.  Otherwise one would need to have the assignment "h = host" in a CODE: or INIT:
       section.)

       The IN/OUTLIST/IN_OUTLIST/OUT/IN_OUT Keywords

       In the list of parameters for an XSUB, one can precede parameter names by the "IN"/"OUT-
       LIST"/"IN_OUTLIST"/"OUT"/"IN_OUT" keywords.  "IN" keyword is the default, the other key-
       words indicate how the Perl interface should differ from the C interface.

       Parameters preceded by "OUTLIST"/"IN_OUTLIST"/"OUT"/"IN_OUT" keywords are considered to be
       used by the C subroutine via pointers.  "OUTLIST"/"OUT" keywords indicate that the C sub-
       routine does not inspect the memory pointed by this parameter, but will write through this
       pointer to provide additional return values.

       Parameters preceded by "OUTLIST" keyword do not appear in the usage signature of the gen-
       erated Perl function.

       Parameters preceded by "IN_OUTLIST"/"IN_OUT"/"OUT" do appear as parameters to the Perl
       function.  With the exception of "OUT"-parameters, these parameters are converted to the
       corresponding C type, then pointers to these data are given as arguments to the C func-
       tion.  It is expected that the C function will write through these pointers.

       The return list of the generated Perl function consists of the C return value from the
       function (unless the XSUB is of "void" return type or "The NO_OUTPUT Keyword" was used)
       followed by all the "OUTLIST" and "IN_OUTLIST" parameters (in the order of appearance).
       On the return from the XSUB the "IN_OUT"/"OUT" Perl parameter will be modified to have the
       values written by the C function.

       For example, an XSUB

	 void
	 day_month(OUTLIST day, IN unix_time, OUTLIST month)
	   int day
	   int unix_time
	   int month

       should be used from Perl as

	 my ($day, $month) = day_month(time);

       The C signature of the corresponding function should be

	 void day_month(int *day, int unix_time, int *month);

       The "IN"/"OUTLIST"/"IN_OUTLIST"/"IN_OUT"/"OUT" keywords can be mixed with ANSI-style dec-
       larations, as in

	 void
	 day_month(OUTLIST int day, int unix_time, OUTLIST int month)

       (here the optional "IN" keyword is omitted).

       The "IN_OUT" parameters are identical with parameters introduced with "The & Unary Opera-
       tor" and put into the "OUTPUT:" section (see "The OUTPUT: Keyword").  The "IN_OUTLIST"
       parameters are very similar, the only difference being that the value C function writes
       through the pointer would not modify the Perl parameter, but is put in the output list.

       The "OUTLIST"/"OUT" parameter differ from "IN_OUTLIST"/"IN_OUT" parameters only by the
       initial value of the Perl parameter not being read (and not being given to the C function
       - which gets some garbage instead).  For example, the same C function as above can be
       interfaced with as

	 void day_month(OUT int day, int unix_time, OUT int month);

       or

	 void
	 day_month(day, unix_time, month)
	     int &day = NO_INIT
	     int  unix_time
	     int &month = NO_INIT
	   OUTPUT:
	     day
	     month

       However, the generated Perl function is called in very C-ish style:

	 my ($day, $month);
	 day_month($day, time, $month);

       The "length(NAME)" Keyword

       If one of the input arguments to the C function is the length of a string argument "NAME",
       one can substitute the name of the length-argument by "length(NAME)" in the XSUB declara-
       tion.  This argument must be omitted when the generated Perl function is called.  E.g.,

	 void
	 dump_chars(char *s, short l)
	 {
	   short n = 0;
	   while (n < l) {
	       printf("s[%d] = \"\\%#03o\"\n", n, (int)s[n]);
	       n++;
	   }
	 }

	 MODULE = x	       PACKAGE = x

	 void dump_chars(char *s, short length(s))

       should be called as "dump_chars($string)".

       This directive is supported with ANSI-type function declarations only.

       Variable-length Parameter Lists

       XSUBs can have variable-length parameter lists by specifying an ellipsis "(...)" in the
       parameter list.	This use of the ellipsis is similar to that found in ANSI C.  The pro-
       grammer is able to determine the number of arguments passed to the XSUB by examining the
       "items" variable which the xsubpp compiler supplies for all XSUBs.  By using this mecha-
       nism one can create an XSUB which accepts a list of parameters of unknown length.

       The host parameter for the rpcb_gettime() XSUB can be optional so the ellipsis can be used
       to indicate that the XSUB will take a variable number of parameters.  Perl should be able
       to call this XSUB with either of the following statements.

	    $status = rpcb_gettime( $timep, $host );

	    $status = rpcb_gettime( $timep );

       The XS code, with ellipsis, follows.

	    bool_t
	    rpcb_gettime(timep, ...)
		 time_t timep = NO_INIT
	       PREINIT:
		 char *host = "localhost";
	       CODE:
		 if( items > 1 )
		      host = (char *)SvPV_nolen(ST(1));
		 RETVAL = rpcb_gettime( host, &timep );
	       OUTPUT:
		 timep
		 RETVAL

       The C_ARGS: Keyword

       The C_ARGS: keyword allows creating of XSUBS which have different calling sequence from
       Perl than from C, without a need to write CODE: or PPCODE: section.  The contents of the
       C_ARGS: paragraph is put as the argument to the called C function without any change.

       For example, suppose that a C function is declared as

	   symbolic nth_derivative(int n, symbolic function, int flags);

       and that the default flags are kept in a global C variable "default_flags".  Suppose that
       you want to create an interface which is called as

	   $second_deriv = $function->nth_derivative(2);

       To do this, declare the XSUB as

	   symbolic
	   nth_derivative(function, n)
	       symbolic        function
	       int	       n
	     C_ARGS:
	       n, function, default_flags

       The PPCODE: Keyword

       The PPCODE: keyword is an alternate form of the CODE: keyword and is used to tell the
       xsubpp compiler that the programmer is supplying the code to control the argument stack
       for the XSUBs return values.  Occasionally one will want an XSUB to return a list of val-
       ues rather than a single value.	In these cases one must use PPCODE: and then explicitly
       push the list of values on the stack.  The PPCODE: and CODE:  keywords should not be used
       together within the same XSUB.

       The actual difference between PPCODE: and CODE: sections is in the initialization of "SP"
       macro (which stands for the current Perl stack pointer), and in the handling of data on
       the stack when returning from an XSUB.  In CODE: sections SP preserves the value which was
       on entry to the XSUB: SP is on the function pointer (which follows the last parameter).
       In PPCODE: sections SP is moved backward to the beginning of the parameter list, which
       allows "PUSH*()" macros to place output values in the place Perl expects them to be when
       the XSUB returns back to Perl.

       The generated trailer for a CODE: section ensures that the number of return values Perl
       will see is either 0 or 1 (depending on the "void"ness of the return value of the C func-
       tion, and heuristics mentioned in "The RETVAL Variable").  The trailer generated for a
       PPCODE: section is based on the number of return values and on the number of times "SP"
       was updated by "[X]PUSH*()" macros.

       Note that macros ST(i), "XST_m*()" and "XSRETURN*()" work equally well in CODE: sections
       and PPCODE: sections.

       The following XSUB will call the C rpcb_gettime() function and will return its two output
       values, timep and status, to Perl as a single list.

	    void
	    rpcb_gettime(host)
		 char *host
	       PREINIT:
		 time_t  timep;
		 bool_t  status;
	       PPCODE:
		 status = rpcb_gettime( host, &timep );
		 EXTEND(SP, 2);
		 PUSHs(sv_2mortal(newSViv(status)));
		 PUSHs(sv_2mortal(newSViv(timep)));

       Notice that the programmer must supply the C code necessary to have the real rpcb_get-
       time() function called and to have the return values properly placed on the argument
       stack.

       The "void" return type for this function tells the xsubpp compiler that the RETVAL vari-
       able is not needed or used and that it should not be created.  In most scenarios the void
       return type should be used with the PPCODE: directive.

       The EXTEND() macro is used to make room on the argument stack for 2 return values.  The
       PPCODE: directive causes the xsubpp compiler to create a stack pointer available as "SP",
       and it is this pointer which is being used in the EXTEND() macro.  The values are then
       pushed onto the stack with the PUSHs() macro.

       Now the rpcb_gettime() function can be used from Perl with the following statement.

	    ($status, $timep) = rpcb_gettime("localhost");

       When handling output parameters with a PPCODE section, be sure to handle 'set' magic prop-
       erly.  See perlguts for details about 'set' magic.

       Returning Undef And Empty Lists

       Occasionally the programmer will want to return simply "undef" or an empty list if a func-
       tion fails rather than a separate status value.	The rpcb_gettime() function offers just
       this situation.	If the function succeeds we would like to have it return the time and if
       it fails we would like to have undef returned.  In the following Perl code the value of
       $timep will either be undef or it will be a valid time.

	    $timep = rpcb_gettime( "localhost" );

       The following XSUB uses the "SV *" return type as a mnemonic only, and uses a CODE: block
       to indicate to the compiler that the programmer has supplied all the necessary code.  The
       sv_newmortal() call will initialize the return value to undef, making that the default
       return value.

	    SV *
	    rpcb_gettime(host)
		 char *  host
	       PREINIT:
		 time_t  timep;
		 bool_t x;
	       CODE:
		 ST(0) = sv_newmortal();
		 if( rpcb_gettime( host, &timep ) )
		      sv_setnv( ST(0), (double)timep);

       The next example demonstrates how one would place an explicit undef in the return value,
       should the need arise.

	    SV *
	    rpcb_gettime(host)
		 char *  host
	       PREINIT:
		 time_t  timep;
		 bool_t x;
	       CODE:
		 if( rpcb_gettime( host, &timep ) ){
		      ST(0) = sv_newmortal();
		      sv_setnv( ST(0), (double)timep);
		 }
		 else{
		      ST(0) = &PL_sv_undef;
		 }

       To return an empty list one must use a PPCODE: block and then not push return values on
       the stack.

	    void
	    rpcb_gettime(host)
		 char *host
	       PREINIT:
		 time_t  timep;
	       PPCODE:
		 if( rpcb_gettime( host, &timep ) )
		      PUSHs(sv_2mortal(newSViv(timep)));
		 else{
		     /* Nothing pushed on stack, so an empty
		      * list is implicitly returned. */
		 }

       Some people may be inclined to include an explicit "return" in the above XSUB, rather than
       letting control fall through to the end.  In those situations "XSRETURN_EMPTY" should be
       used, instead.  This will ensure that the XSUB stack is properly adjusted.  Consult per-
       lapi for other "XSRETURN" macros.

       Since "XSRETURN_*" macros can be used with CODE blocks as well, one can rewrite this exam-
       ple as:

	    int
	    rpcb_gettime(host)
		 char *host
	       PREINIT:
		 time_t  timep;
	       CODE:
		 RETVAL = rpcb_gettime( host, &timep );
		 if (RETVAL == 0)
		       XSRETURN_UNDEF;
	       OUTPUT:
		 RETVAL

       In fact, one can put this check into a POSTCALL: section as well.  Together with PREINIT:
       simplifications, this leads to:

	    int
	    rpcb_gettime(host)
		 char *host
		 time_t  timep;
	       POSTCALL:
		 if (RETVAL == 0)
		       XSRETURN_UNDEF;

       The REQUIRE: Keyword

       The REQUIRE: keyword is used to indicate the minimum version of the xsubpp compiler needed
       to compile the XS module.  An XS module which contains the following statement will com-
       pile with only xsubpp version 1.922 or greater:

	       REQUIRE: 1.922

       The CLEANUP: Keyword

       This keyword can be used when an XSUB requires special cleanup procedures before it termi-
       nates.  When the CLEANUP:  keyword is used it must follow any CODE:, PPCODE:, or OUTPUT:
       blocks which are present in the XSUB.  The code specified for the cleanup block will be
       added as the last statements in the XSUB.

       The POSTCALL: Keyword

       This keyword can be used when an XSUB requires special procedures executed after the C
       subroutine call is performed.  When the POSTCALL: keyword is used it must precede OUTPUT:
       and CLEANUP: blocks which are present in the XSUB.

       See examples in "The NO_OUTPUT Keyword" and "Returning Undef And Empty Lists".

       The POSTCALL: block does not make a lot of sense when the C subroutine call is supplied by
       user by providing either CODE: or PPCODE: section.

       The BOOT: Keyword

       The BOOT: keyword is used to add code to the extension's bootstrap function.  The boot-
       strap function is generated by the xsubpp compiler and normally holds the statements nec-
       essary to register any XSUBs with Perl.	With the BOOT: keyword the programmer can tell
       the compiler to add extra statements to the bootstrap function.

       This keyword may be used any time after the first MODULE keyword and should appear on a
       line by itself.	The first blank line after the keyword will terminate the code block.

	    BOOT:
	    # The following message will be printed when the
	    # bootstrap function executes.
	    printf("Hello from the bootstrap!\n");

       The VERSIONCHECK: Keyword

       The VERSIONCHECK: keyword corresponds to xsubpp's "-versioncheck" and "-noversioncheck"
       options.  This keyword overrides the command line options.  Version checking is enabled by
       default.  When version checking is enabled the XS module will attempt to verify that its
       version matches the version of the PM module.

       To enable version checking:

	   VERSIONCHECK: ENABLE

       To disable version checking:

	   VERSIONCHECK: DISABLE

       The PROTOTYPES: Keyword

       The PROTOTYPES: keyword corresponds to xsubpp's "-prototypes" and "-noprototypes" options.
       This keyword overrides the command line options.  Prototypes are enabled by default.  When
       prototypes are enabled XSUBs will be given Perl prototypes.  This keyword may be used mul-
       tiple times in an XS module to enable and disable prototypes for different parts of the
       module.

       To enable prototypes:

	   PROTOTYPES: ENABLE

       To disable prototypes:

	   PROTOTYPES: DISABLE

       The PROTOTYPE: Keyword

       This keyword is similar to the PROTOTYPES: keyword above but can be used to force xsubpp
       to use a specific prototype for the XSUB.  This keyword overrides all other prototype
       options and keywords but affects only the current XSUB.	Consult "Prototypes" in perlsub
       for information about Perl prototypes.

	   bool_t
	   rpcb_gettime(timep, ...)
		 time_t timep = NO_INIT
	       PROTOTYPE: $;$
	       PREINIT:
		 char *host = "localhost";
	       CODE:
			 if( items > 1 )
			      host = (char *)SvPV_nolen(ST(1));
			 RETVAL = rpcb_gettime( host, &timep );
	       OUTPUT:
		 timep
		 RETVAL

       If the prototypes are enabled, you can disable it locally for a given XSUB as in the fol-
       lowing example:

	   void
	   rpcb_gettime_noproto()
	       PROTOTYPE: DISABLE
	   ...

       The ALIAS: Keyword

       The ALIAS: keyword allows an XSUB to have two or more unique Perl names and to know which
       of those names was used when it was invoked.  The Perl names may be fully-qualified with
       package names.  Each alias is given an index.  The compiler will setup a variable called
       "ix" which contain the index of the alias which was used.  When the XSUB is called with
       its declared name "ix" will be 0.

       The following example will create aliases "FOO::gettime()" and "BAR::getit()" for this
       function.

	   bool_t
	   rpcb_gettime(host,timep)
		 char *host
		 time_t &timep
	       ALIAS:
		   FOO::gettime = 1
		   BAR::getit = 2
	       INIT:
		 printf("# ix = %d\n", ix );
	       OUTPUT:
		 timep

       The OVERLOAD: Keyword

       Instead of writing an overloaded interface using pure Perl, you can also use the OVERLOAD
       keyword to define additional Perl names for your functions (like the ALIAS: keyword
       above).	However, the overloaded functions must be defined with three parameters (except
       for the nomethod() function which needs four parameters).  If any function has the OVER-
       LOAD: keyword, several additional lines will be defined in the c file generated by xsubpp
       in order to register with the overload magic.

       Since blessed objects are actually stored as RV's, it is useful to use the typemap fea-
       tures to preprocess parameters and extract the actual SV stored within the blessed RV. See
       the sample for T_PTROBJ_SPECIAL below.

       To use the OVERLOAD: keyword, create an XS function which takes three input parameters (
       or use the c style '...' definition) like this:

	   SV *
	   cmp (lobj, robj, swap)
	   My_Module_obj    lobj
	   My_Module_obj    robj
	   IV		    swap
	   OVERLOAD: cmp <=>
	   { /* function defined here */}

       In this case, the function will overload both of the three way comparison operators.  For
       all overload operations using non-alpha characters, you must type the parameter without
       quoting, separating multiple overloads with whitespace.	Note that "" (the stringify over-
       load) should be entered as \"\" (i.e. escaped).

       The FALLBACK: Keyword

       In addition to the OVERLOAD keyword, if you need to control how Perl autogenerates missing
       overloaded operators, you can set the FALLBACK keyword in the module header section, like
       this:

	   MODULE = RPC  PACKAGE = RPC

	   FALLBACK: TRUE
	   ...

       where FALLBACK can take any of the three values TRUE, FALSE, or UNDEF.  If you do not set
       any FALLBACK value when using OVERLOAD, it defaults to UNDEF.  FALLBACK is not used except
       when one or more functions using OVERLOAD have been defined.  Please see "Fallback" in
       overload for more details.

       The INTERFACE: Keyword

       This keyword declares the current XSUB as a keeper of the given calling signature.  If
       some text follows this keyword, it is considered as a list of functions which have this
       signature, and should be attached to the current XSUB.

       For example, if you have 4 C functions multiply(), divide(), add(), subtract() all having
       the signature:

	   symbolic f(symbolic, symbolic);

       you can make them all to use the same XSUB using this:

	   symbolic
	   interface_s_ss(arg1, arg2)
	       symbolic        arg1
	       symbolic        arg2
	   INTERFACE:
	       multiply divide
	       add subtract

       (This is the complete XSUB code for 4 Perl functions!)  Four generated Perl function share
       names with corresponding C functions.

       The advantage of this approach comparing to ALIAS: keyword is that there is no need to
       code a switch statement, each Perl function (which shares the same XSUB) knows which C
       function it should call.  Additionally, one can attach an extra function remainder() at
       runtime by using

	   CV *mycv = newXSproto("Symbolic::remainder",
				 XS_Symbolic_interface_s_ss, __FILE__, "$$");
	   XSINTERFACE_FUNC_SET(mycv, remainder);

       say, from another XSUB.	(This example supposes that there was no INTERFACE_MACRO: sec-
       tion, otherwise one needs to use something else instead of "XSINTERFACE_FUNC_SET", see the
       next section.)

       The INTERFACE_MACRO: Keyword

       This keyword allows one to define an INTERFACE using a different way to extract a function
       pointer from an XSUB.  The text which follows this keyword should give the name of macros
       which would extract/set a function pointer.  The extractor macro is given return type,
       "CV*", and "XSANY.any_dptr" for this "CV*".  The setter macro is given cv, and the func-
       tion pointer.

       The default value is "XSINTERFACE_FUNC" and "XSINTERFACE_FUNC_SET".  An INTERFACE keyword
       with an empty list of functions can be omitted if INTERFACE_MACRO keyword is used.

       Suppose that in the previous example functions pointers for multiply(), divide(), add(),
       subtract() are kept in a global C array "fp[]" with offsets being "multiply_off",
       "divide_off", "add_off", "subtract_off".  Then one can use

	   #define XSINTERFACE_FUNC_BYOFFSET(ret,cv,f) \
	       ((XSINTERFACE_CVT_ANON(ret))fp[CvXSUBANY(cv).any_i32])
	   #define XSINTERFACE_FUNC_BYOFFSET_set(cv,f) \
	       CvXSUBANY(cv).any_i32 = CAT2( f, _off )

       in C section,

	   symbolic
	   interface_s_ss(arg1, arg2)
	       symbolic        arg1
	       symbolic        arg2
	     INTERFACE_MACRO:
	       XSINTERFACE_FUNC_BYOFFSET
	       XSINTERFACE_FUNC_BYOFFSET_set
	     INTERFACE:
	       multiply divide
	       add subtract

       in XSUB section.

       The INCLUDE: Keyword

       This keyword can be used to pull other files into the XS module.  The other files may have
       XS code.  INCLUDE: can also be used to run a command to generate the XS code to be pulled
       into the module.

       The file Rpcb1.xsh contains our "rpcb_gettime()" function:

	   bool_t
	   rpcb_gettime(host,timep)
		 char *host
		 time_t &timep
	       OUTPUT:
		 timep

       The XS module can use INCLUDE: to pull that file into it.

	   INCLUDE: Rpcb1.xsh

       If the parameters to the INCLUDE: keyword are followed by a pipe ("|") then the compiler
       will interpret the parameters as a command.

	   INCLUDE: cat Rpcb1.xsh |

       The CASE: Keyword

       The CASE: keyword allows an XSUB to have multiple distinct parts with each part acting as
       a virtual XSUB.	CASE: is greedy and if it is used then all other XS keywords must be con-
       tained within a CASE:.  This means nothing may precede the first CASE: in the XSUB and
       anything following the last CASE: is included in that case.

       A CASE: might switch via a parameter of the XSUB, via the "ix" ALIAS: variable (see "The
       ALIAS: Keyword"), or maybe via the "items" variable (see "Variable-length Parameter
       Lists").  The last CASE: becomes the default case if it is not associated with a condi-
       tional.	The following example shows CASE switched via "ix" with a function "rpcb_get-
       time()" having an alias "x_gettime()".  When the function is called as "rpcb_gettime()"
       its parameters are the usual "(char *host, time_t *timep)", but when the function is
       called as "x_gettime()" its parameters are reversed, "(time_t *timep, char *host)".

	   long
	   rpcb_gettime(a,b)
	     CASE: ix == 1
	       ALIAS:
		 x_gettime = 1
	       INPUT:
		 # 'a' is timep, 'b' is host
		 char *b
		 time_t a = NO_INIT
	       CODE:
		      RETVAL = rpcb_gettime( b, &a );
	       OUTPUT:
		 a
		 RETVAL
	     CASE:
		 # 'a' is host, 'b' is timep
		 char *a
		 time_t &b = NO_INIT
	       OUTPUT:
		 b
		 RETVAL

       That function can be called with either of the following statements.  Note the different
       argument lists.

	       $status = rpcb_gettime( $host, $timep );

	       $status = x_gettime( $timep, $host );

       The & Unary Operator

       The "&" unary operator in the INPUT: section is used to tell xsubpp that it should convert
       a Perl value to/from C using the C type to the left of "&", but provide a pointer to this
       value when the C function is called.

       This is useful to avoid a CODE: block for a C function which takes a parameter by refer-
       ence.  Typically, the parameter should be not a pointer type (an "int" or "long" but not
       an "int*" or "long*").

       The following XSUB will generate incorrect C code.  The xsubpp compiler will turn this
       into code which calls "rpcb_gettime()" with parameters "(char *host, time_t timep)", but
       the real "rpcb_gettime()" wants the "timep" parameter to be of type "time_t*" rather than
       "time_t".

	   bool_t
	   rpcb_gettime(host,timep)
		 char *host
		 time_t timep
	       OUTPUT:
		 timep

       That problem is corrected by using the "&" operator.  The xsubpp compiler will now turn
       this into code which calls "rpcb_gettime()" correctly with parameters "(char *host, time_t
       *timep)".  It does this by carrying the "&" through, so the function call looks like
       "rpcb_gettime(host, &timep)".

	   bool_t
	   rpcb_gettime(host,timep)
		 char *host
		 time_t &timep
	       OUTPUT:
		 timep

       Inserting POD, Comments and C Preprocessor Directives

       C preprocessor directives are allowed within BOOT:, PREINIT: INIT:, CODE:, PPCODE:, POST-
       CALL:, and CLEANUP: blocks, as well as outside the functions.  Comments are allowed any-
       where after the MODULE keyword.	The compiler will pass the preprocessor directives
       through untouched and will remove the commented lines. POD documentation is allowed at any
       point, both in the C and XS language sections. POD must be terminated with a "=cut" com-
       mand; "xsubpp" will exit with an error if it does not. It is very unlikely that human gen-
       erated C code will be mistaken for POD, as most indenting styles result in whitespace in
       front of any line starting with "=". Machine generated XS files may fall into this trap
       unless care is taken to ensure that a space breaks the sequence "\n=".

       Comments can be added to XSUBs by placing a "#" as the first non-whitespace of a line.
       Care should be taken to avoid making the comment look like a C preprocessor directive,
       lest it be interpreted as such.	The simplest way to prevent this is to put whitespace in
       front of the "#".

       If you use preprocessor directives to choose one of two versions of a function, use

	   #if ... version1
	   #else /* ... version2  */
	   #endif

       and not

	   #if ... version1
	   #endif
	   #if ... version2
	   #endif

       because otherwise xsubpp will believe that you made a duplicate definition of the func-
       tion.  Also, put a blank line before the #else/#endif so it will not be seen as part of
       the function body.

       Using XS With C++

       If an XSUB name contains "::", it is considered to be a C++ method.  The generated Perl
       function will assume that its first argument is an object pointer.  The object pointer
       will be stored in a variable called THIS.  The object should have been created by C++ with
       the new() function and should be blessed by Perl with the sv_setref_pv() macro.	The
       blessing of the object by Perl can be handled by a typemap.  An example typemap is shown
       at the end of this section.

       If the return type of the XSUB includes "static", the method is considered to be a static
       method.	It will call the C++ function using the class::method() syntax.  If the method is
       not static the function will be called using the THIS->method() syntax.

       The next examples will use the following C++ class.

	    class color {
		 public:
		 color();
		 ~color();
		 int blue();
		 void set_blue( int );

		 private:
		 int c_blue;
	    };

       The XSUBs for the blue() and set_blue() methods are defined with the class name but the
       parameter for the object (THIS, or "self") is implicit and is not listed.

	    int
	    color::blue()

	    void
	    color::set_blue( val )
		 int val

       Both Perl functions will expect an object as the first parameter.  In the generated C++
       code the object is called "THIS", and the method call will be performed on this object.
       So in the C++ code the blue() and set_blue() methods will be called as this:

	    RETVAL = THIS->blue();

	    THIS->set_blue( val );

       You could also write a single get/set method using an optional argument:

	    int
	    color::blue( val = NO_INIT )
		int val
		PROTOTYPE $;$
		CODE:
		    if (items > 1)
			THIS->set_blue( val );
		    RETVAL = THIS->blue();
		OUTPUT:
		    RETVAL

       If the function's name is DESTROY then the C++ "delete" function will be called and "THIS"
       will be given as its parameter.	The generated C++ code for

	    void
	    color::DESTROY()

       will look like this:

	    color *THIS = ...; // Initialized as in typemap

	    delete THIS;

       If the function's name is new then the C++ "new" function will be called to create a
       dynamic C++ object.  The XSUB will expect the class name, which will be kept in a variable
       called "CLASS", to be given as the first argument.

	    color *
	    color::new()

       The generated C++ code will call "new".

	    RETVAL = new color();

       The following is an example of a typemap that could be used for this C++ example.

	   TYPEMAP
	   color *	       O_OBJECT

	   OUTPUT
	   # The Perl object is blessed into 'CLASS', which should be a
	   # char* having the name of the package for the blessing.
	   O_OBJECT
	       sv_setref_pv( $arg, CLASS, (void*)$var );

	   INPUT
	   O_OBJECT
	       if( sv_isobject($arg) && (SvTYPE(SvRV($arg)) == SVt_PVMG) )
		       $var = ($type)SvIV((SV*)SvRV( $arg ));
	       else{
		       warn( \"${Package}::$func_name() -- $var is not a blessed SV reference\" );
		       XSRETURN_UNDEF;
	       }

       Interface Strategy

       When designing an interface between Perl and a C library a straight translation from C to
       XS (such as created by "h2xs -x") is often sufficient.  However, sometimes the interface
       will look very C-like and occasionally nonintuitive, especially when the C function modi-
       fies one of its parameters, or returns failure inband (as in "negative return values mean
       failure").  In cases where the programmer wishes to create a more Perl-like interface the
       following strategy may help to identify the more critical parts of the interface.

       Identify the C functions with input/output or output parameters.  The XSUBs for these
       functions may be able to return lists to Perl.

       Identify the C functions which use some inband info as an indication of failure.  They may
       be candidates to return undef or an empty list in case of failure.  If the failure may be
       detected without a call to the C function, you may want to use an INIT: section to report
       the failure.  For failures detectable after the C function returns one may want to use a
       POSTCALL: section to process the failure.  In more complicated cases use CODE: or PPCODE:
       sections.

       If many functions use the same failure indication based on the return value, you may want
       to create a special typedef to handle this situation.  Put

	 typedef int negative_is_failure;

       near the beginning of XS file, and create an OUTPUT typemap entry for "negative_is_fail-
       ure" which converts negative values to "undef", or maybe croak()s.  After this the return
       value of type "negative_is_failure" will create more Perl-like interface.

       Identify which values are used by only the C and XSUB functions themselves, say, when a
       parameter to a function should be a contents of a global variable.  If Perl does not need
       to access the contents of the value then it may not be necessary to provide a translation
       for that value from C to Perl.

       Identify the pointers in the C function parameter lists and return values.  Some pointers
       may be used to implement input/output or output parameters, they can be handled in XS with
       the "&" unary operator, and, possibly, using the NO_INIT keyword.  Some others will
       require handling of types like "int *", and one needs to decide what a useful Perl trans-
       lation will do in such a case.  When the semantic is clear, it is advisable to put the
       translation into a typemap file.

       Identify the structures used by the C functions.  In many cases it may be helpful to use
       the T_PTROBJ typemap for these structures so they can be manipulated by Perl as blessed
       objects.  (This is handled automatically by "h2xs -x".)

       If the same C type is used in several different contexts which require different transla-
       tions, "typedef" several new types mapped to this C type, and create separate typemap
       entries for these new types.  Use these types in declarations of return type and parame-
       ters to XSUBs.

       Perl Objects And C Structures

       When dealing with C structures one should select either T_PTROBJ or T_PTRREF for the XS
       type.  Both types are designed to handle pointers to complex objects.  The T_PTRREF type
       will allow the Perl object to be unblessed while the T_PTROBJ type requires that the
       object be blessed.  By using T_PTROBJ one can achieve a form of type-checking because the
       XSUB will attempt to verify that the Perl object is of the expected type.

       The following XS code shows the getnetconfigent() function which is used with ONC+ TIRPC.
       The getnetconfigent() function will return a pointer to a C structure and has the C proto-
       type shown below.  The example will demonstrate how the C pointer will become a Perl ref-
       erence.	Perl will consider this reference to be a pointer to a blessed object and will
       attempt to call a destructor for the object.  A destructor will be provided in the XS
       source to free the memory used by getnetconfigent().  Destructors in XS can be created by
       specifying an XSUB function whose name ends with the word DESTROY.  XS destructors can be
       used to free memory which may have been malloc'd by another XSUB.

	    struct netconfig *getnetconfigent(const char *netid);

       A "typedef" will be created for "struct netconfig".  The Perl object will be blessed in a
       class matching the name of the C type, with the tag "Ptr" appended, and the name should
       not have embedded spaces if it will be a Perl package name.  The destructor will be placed
       in a class corresponding to the class of the object and the PREFIX keyword will be used to
       trim the name to the word DESTROY as Perl will expect.

	    typedef struct netconfig Netconfig;

	    MODULE = RPC  PACKAGE = RPC

	    Netconfig *
	    getnetconfigent(netid)
		 char *netid

	    MODULE = RPC  PACKAGE = NetconfigPtr  PREFIX = rpcb_

	    void
	    rpcb_DESTROY(netconf)
		 Netconfig *netconf
	       CODE:
		 printf("Now in NetconfigPtr::DESTROY\n");
		 free( netconf );

       This example requires the following typemap entry.  Consult the typemap section for more
       information about adding new typemaps for an extension.

	    TYPEMAP
	    Netconfig *  T_PTROBJ

       This example will be used with the following Perl statements.

	    use RPC;
	    $netconf = getnetconfigent("udp");

       When Perl destroys the object referenced by $netconf it will send the object to the sup-
       plied XSUB DESTROY function.  Perl cannot determine, and does not care, that this object
       is a C struct and not a Perl object.  In this sense, there is no difference between the
       object created by the getnetconfigent() XSUB and an object created by a normal Perl sub-
       routine.

       The Typemap

       The typemap is a collection of code fragments which are used by the xsubpp compiler to map
       C function parameters and values to Perl values.  The typemap file may consist of three
       sections labelled "TYPEMAP", "INPUT", and "OUTPUT".  An unlabelled initial section is
       assumed to be a "TYPEMAP" section.  The INPUT section tells the compiler how to translate
       Perl values into variables of certain C types.  The OUTPUT section tells the compiler how
       to translate the values from certain C types into values Perl can understand.  The TYPEMAP
       section tells the compiler which of the INPUT and OUTPUT code fragments should be used to
       map a given C type to a Perl value.  The section labels "TYPEMAP", "INPUT", or "OUTPUT"
       must begin in the first column on a line by themselves, and must be in uppercase.

       The default typemap in the "lib/ExtUtils" directory of the Perl source contains many use-
       ful types which can be used by Perl extensions.	Some extensions define additional
       typemaps which they keep in their own directory.  These additional typemaps may reference
       INPUT and OUTPUT maps in the main typemap.  The xsubpp compiler will allow the extension's
       own typemap to override any mappings which are in the default typemap.

       Most extensions which require a custom typemap will need only the TYPEMAP section of the
       typemap file.  The custom typemap used in the getnetconfigent() example shown earlier
       demonstrates what may be the typical use of extension typemaps.	That typemap is used to
       equate a C structure with the T_PTROBJ typemap.	The typemap used by getnetconfigent() is
       shown here.  Note that the C type is separated from the XS type with a tab and that the C
       unary operator "*" is considered to be a part of the C type name.

	       TYPEMAP
	       Netconfig *<tab>T_PTROBJ

       Here's a more complicated example: suppose that you wanted "struct netconfig" to be
       blessed into the class "Net::Config".  One way to do this is to use underscores (_) to
       separate package names, as follows:

	       typedef struct netconfig * Net_Config;

       And then provide a typemap entry "T_PTROBJ_SPECIAL" that maps underscores to double-colons
       (::), and declare "Net_Config" to be of that type:

	       TYPEMAP
	       Net_Config      T_PTROBJ_SPECIAL

	       INPUT
	       T_PTROBJ_SPECIAL
		       if (sv_derived_from($arg, \"${(my $ntt=$ntype)=~s/_/::/g;\$ntt}\")) {
			       IV tmp = SvIV((SV*)SvRV($arg));
			       $var = INT2PTR($type, tmp);
		       }
		       else
			       croak(\"$var is not of type ${(my $ntt=$ntype)=~s/_/::/g;\$ntt}\")

	       OUTPUT
	       T_PTROBJ_SPECIAL
		       sv_setref_pv($arg, \"${(my $ntt=$ntype)=~s/_/::/g;\$ntt}\",
		       (void*)$var);

       The INPUT and OUTPUT sections substitute underscores for double-colons on the fly, giving
       the desired effect.  This example demonstrates some of the power and versatility of the
       typemap facility.

       The INT2PTR macro (defined in perl.h) casts an integer to a pointer, of a given type, tak-
       ing care of the possible different size of integers and pointers.  There are also PTR2IV,
       PTR2UV, PTR2NV macros, to map the other way, which may be useful in OUTPUT sections.

       Safely Storing Static Data in XS

       Starting with Perl 5.8, a macro framework has been defined to allow static data to be
       safely stored in XS modules that will be accessed from a multi-threaded Perl.

       Although primarily designed for use with multi-threaded Perl, the macros have been
       designed so that they will work with non-threaded Perl as well.

       It is therefore strongly recommended that these macros be used by all XS modules that make
       use of static data.

       The easiest way to get a template set of macros to use is by specifying the "-g"
       ("--global") option with h2xs (see h2xs).

       Below is an example module that makes use of the macros.

	   #include "EXTERN.h"
	   #include "perl.h"
	   #include "XSUB.h"

	   /* Global Data */

	   #define MY_CXT_KEY "BlindMice::_guts" XS_VERSION

	   typedef struct {
	       int count;
	       char name[3][100];
	   } my_cxt_t;

	   START_MY_CXT

	   MODULE = BlindMice		PACKAGE = BlindMice

	   BOOT:
	   {
	       MY_CXT_INIT;
	       MY_CXT.count = 0;
	       strcpy(MY_CXT.name[0], "None");
	       strcpy(MY_CXT.name[1], "None");
	       strcpy(MY_CXT.name[2], "None");
	   }

	   int
	   newMouse(char * name)
	       char * name;
	       PREINIT:
		 dMY_CXT;
	       CODE:
		 if (MY_CXT.count >= 3) {
		     warn("Already have 3 blind mice");
		     RETVAL = 0;
		 }
		 else {
		     RETVAL = ++ MY_CXT.count;
		     strcpy(MY_CXT.name[MY_CXT.count - 1], name);
		 }

	   char *
	   get_mouse_name(index)
	     int index
	     CODE:
	       dMY_CXT;
	       RETVAL = MY_CXT.lives ++;
	       if (index > MY_CXT.count)
		 croak("There are only 3 blind mice.");
	       else
		 RETVAL = newSVpv(MY_CXT.name[index - 1]);

	   void
	   CLONE(...)
	       CODE:
	       MY_CXT_CLONE;

       REFERENCE

       MY_CXT_KEY
	    This macro is used to define a unique key to refer to the static data for an XS mod-
	    ule. The suggested naming scheme, as used by h2xs, is to use a string that consists
	    of the module name, the string "::_guts" and the module version number.

		#define MY_CXT_KEY "MyModule::_guts" XS_VERSION

       typedef my_cxt_t
	    This struct typedef must always be called "my_cxt_t" -- the other "CXT*" macros
	    assume the existence of the "my_cxt_t" typedef name.

	    Declare a typedef named "my_cxt_t" that is a structure that contains all the data
	    that needs to be interpreter-local.

		typedef struct {
		    int some_value;
		} my_cxt_t;

       START_MY_CXT
	    Always place the START_MY_CXT macro directly after the declaration of "my_cxt_t".

       MY_CXT_INIT
	    The MY_CXT_INIT macro initialises storage for the "my_cxt_t" struct.

	    It must be called exactly once -- typically in a BOOT: section. If you are maintain-
	    ing multiple interpreters, it should be called once in each interpreter instance,
	    except for interpreters cloned from existing ones.	(But see "MY_CXT_CLONE" below.)

       dMY_CXT
	    Use the dMY_CXT macro (a declaration) in all the functions that access MY_CXT.

       MY_CXT
	    Use the MY_CXT macro to access members of the "my_cxt_t" struct. For example, if
	    "my_cxt_t" is

		typedef struct {
		    int index;
		} my_cxt_t;

	    then use this to access the "index" member

		dMY_CXT;
		MY_CXT.index = 2;

       aMY_CXT/pMY_CXT
	    "dMY_CXT" may be quite expensive to calculate, and to avoid the overhead of invoking
	    it in each function it is possible to pass the declaration onto other functions using
	    the "aMY_CXT"/"pMY_CXT" macros, eg

		void sub1() {
		    dMY_CXT;
		    MY_CXT.index = 1;
		    sub2(aMY_CXT);
		}

		void sub2(pMY_CXT) {
		    MY_CXT.index = 2;
		}

	    Analogously to "pTHX", there are equivalent forms for when the macro is the first or
	    last in multiple arguments, where an underscore represents a comma, i.e.  "_aMY_CXT",
	    "aMY_CXT_", "_pMY_CXT" and "pMY_CXT_".

       MY_CXT_CLONE
	    By default, when a new interpreter is created as a copy of an existing one (eg via
	    "threads->create()"), both interpreters share the same physical my_cxt_t structure.
	    Calling "MY_CXT_CLONE" (typically via the package's "CLONE()" function), causes a
	    byte-for-byte copy of the structure to be taken, and any future dMY_CXT will cause
	    the copy to be accessed instead.

       Thread-aware system interfaces

       Starting from Perl 5.8, in C/C++ level Perl knows how to wrap system/library interfaces
       that have thread-aware versions (e.g. getpwent_r()) into frontend macros (e.g. getpwent())
       that correctly handle the multithreaded interaction with the Perl interpreter.  This will
       happen transparently, the only thing you need to do is to instantiate a Perl interpreter.

       This wrapping happens always when compiling Perl core source (PERL_CORE is defined) or the
       Perl core extensions (PERL_EXT is defined).  When compiling XS code outside of Perl core
       the wrapping does not take place.  Note, however, that intermixing the _r-forms (as Perl
       compiled for multithreaded operation will do) and the _r-less forms is neither well-
       defined (inconsistent results, data corruption, or even crashes become more likely), nor
       is it very portable.

EXAMPLES
       File "RPC.xs": Interface to some ONC+ RPC bind library functions.

	    #include "EXTERN.h"
	    #include "perl.h"
	    #include "XSUB.h"

	    #include <rpc/rpc.h>

	    typedef struct netconfig Netconfig;

	    MODULE = RPC  PACKAGE = RPC

	    SV *
	    rpcb_gettime(host="localhost")
		 char *host
	       PREINIT:
		 time_t  timep;
	       CODE:
		 ST(0) = sv_newmortal();
		 if( rpcb_gettime( host, &timep ) )
		      sv_setnv( ST(0), (double)timep );

	    Netconfig *
	    getnetconfigent(netid="udp")
		 char *netid

	    MODULE = RPC  PACKAGE = NetconfigPtr  PREFIX = rpcb_

	    void
	    rpcb_DESTROY(netconf)
		 Netconfig *netconf
	       CODE:
		 printf("NetconfigPtr::DESTROY\n");
		 free( netconf );

       File "typemap": Custom typemap for RPC.xs.

	    TYPEMAP
	    Netconfig *  T_PTROBJ

       File "RPC.pm": Perl module for the RPC extension.

	    package RPC;

	    require Exporter;
	    require DynaLoader;
	    @ISA = qw(Exporter DynaLoader);
	    @EXPORT = qw(rpcb_gettime getnetconfigent);

	    bootstrap RPC;
	    1;

       File "rpctest.pl": Perl test program for the RPC extension.

	    use RPC;

	    $netconf = getnetconfigent();
	    $a = rpcb_gettime();
	    print "time = $a\n";
	    print "netconf = $netconf\n";

	    $netconf = getnetconfigent("tcp");
	    $a = rpcb_gettime("poplar");
	    print "time = $a\n";
	    print "netconf = $netconf\n";

XS VERSION
       This document covers features supported by "xsubpp" 1.935.

AUTHOR
       Originally written by Dean Roehrich <roehrich@cray.com>.

       Maintained since 1996 by The Perl Porters <perlbug@perl.org>.

perl v5.8.9				    2007-11-17					PERLXS(1)
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